Conventionally metals have been prevented from corrosion, for example, by treating the surface of metals, i.e. by forming an inorganic coating of phosphoric acid salt, chromic acid salt or the like over the metal surface, or by techniques for forming corrosion preventive coatings, i.e. by forming an organic coating of epoxy resin, phenol resin or the like over the metal surface.
However, the treatment of metal surfaces has the drawbacks of causing environmental pollution due to liquid wastes containing heavy metals, using poisonous chemicals such as acids, alkalis or cyano-containing compounds, and necessitating a complex process.
Corrosion preventive coatings have the problem that the bond strength between the coating and the metal is insufficient to completely prevent the corrosion reaction.
Accordingly, corrosion preventive techniques still remain to be established which are free of pollution, practically useful and excellent in corrosion preventive effects, and it has been desired to develop such techniques.
In application of coating on automotive bodies and electrical equipment, it is widely practiced to pretreat the surface of metal with the phosphoric acid salt, followed by application of an electrophoretic coating composition as a primer in view of resistance to corrosion on metal surfaces, adhesion between the metal surface and the coating, finishing appearance of coating, durability thereof, productivity thereof, etc.
In this case, while the electrophoretic coating composition is substantially pollution-free, the treatment with phosphoric acid salt entails the problems that the phosphorus in the liquid waste from the treatment process induces eutrophication of water, thereby polluting the water, and that a short supply of phosphorus is expected in future. Thus there is an urgent need for development of a novel pretreatment method which can replace the treatment with phosphoric acid salt.